Remove `anyio`-specific code in test code using `ipykernel > 7`

There is some code related to `anyio` here:
https://github.com/jupyter/jupyter_client/blob/eb96748e0afb970c806653824540dbadc83ffc84/tests/signalkernel.py#L62

which I introduced in #1034 to deal with the then intended use of `anyio` in `ipykernel 7`. There will no longer be use of `anyio` code in `ipykernel 7`, so this should be removed.

I'll submit a PR tomorrow.